A process for producing methionine by the hydrolysis of 5-(β-methylmercaptoethyl)hydantoin (hereinafter referred to as “M-hydantoin”) is usually carried out according to the following reaction scheme in the presence of an alkali such as potassium carbonate:

In general, the reaction conditions for this hydrolysis reaction include a pressure of about 0.5 to 1.5 MPaG and a temperature of 150 to 200° C. The corrosion resistance of metallic materials under these hydrolysis conditions and also under conditions for operating associated apparatuses including, for example, preheating for hydrolysis and the treatment of exhaust gas is extremely strict regardless of whether the hydrolysis proceeds in a liquid phase or a gas phase. Therefore, for the construction of the apparatuses, from the viewpoint of having superior corrosion resistance to that of SUS304L stainless steel, austenite chromium-nickel stainless steel, titanium, zirconium, and duplex stainless steel comprising 21.0 to 30.0% by weight of chromium, 4.5 to 11.0% by weight of nickel, 2.5 to 5.0% by weight of molybdenum and 0.05 to 0.35% by weight of nitrogen as chemical components in the steel (cf. JP-A-11-217370) are used. Among them, the stainless steel described in JP-A-11-217370 is often utilized since it is less expensive than titanium and zirconium and has excellent corrosion resistance even at a high temperature. However, in or after the preheating step for hydrolysis, the temperature sometimes rises to 120° C. or higher. In such a case, a solution subjected to the hydrolysis may induce the corrosion of materials, which may result in the corrosion of the metallic materials of the apparatuses and the coloring of products due to metal ions eluted from the metallic materials by corrosion.